HTV arrives at the ISS after extensive on orbit testing

JAXA – the Japanese Space Agency – has successfully debuted their HTV (H-II Transfer Vehicle) on its maiden flight to the International Space Station (ISS), via a series of rendezvous demonstrations and a successful capture via the Space Station Remote Manipulator System (SSRMS). The flight of the HTV has been classed as near-flawless during its on orbit operations.

“HTV1 lifted off on time at 253/17:01:46. Ascent and the upper stage burn were nominal. Separation from the HII-B upper stage occurred at 17:16:52,” noted the opening flight update via NASA’s MOD and acquired by L2.

Only a few issues were noted during the initial operations of the HTV on orbit, as explained by MOD.

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“At approximately 253/17:37, the timer for the earth sensor expired before the earth acquisition was complete. (However,) this timer is only used during the post-insertion activities and should not have any impacts to the rest of the HTV mission.

“The HTV successfully performed three maneuvers today, as follows: PM1 began at 254/00:34:10 and had a dV of 6.64 m/s (performed using Main Engines). M1 began at 254/02:04:08 and had a dV of 0.18 m/s (performed using RCS thrusters). PM1′ began at 254/02:49:01 and had a dV of 0.0075 m/s (performed using RCS thrusters)

“During the attitude transition immediately following the PM1 maneuver, the FDIR (Fault Detection, Isolation and Recovery) triggered a Gyro-A attitude rate comparison error on the Y-axis and the FDIR isolated Gyro-A, after which the HTV performed nominally. (Controllers) subsequently evaluated the performance of Gyro-A and re-integrated it following PM1′. Following re-integration, the HTV performed nominally.

“The engineering team believes the FDIR gyro rate comparison limits are not tuned well for the large Main Engine maneuvers and they are evaluating the options for preventing this limit violation during future main engine maneuvers in attitude mode.”

Several major objectives were required, in order to prove the HTV was capable of a safe arrival at the orbital outpost, as required by any new vehicle approaching Station. This resulted in approval to continue towards Thursday’s rendezvous by the Integrated Mission Management Team (IMMT).

“HTV is GO for Integrated Ops! The IMMT met to review the HTV Demonstration On-orbit Operations Report (H-DOOR) and the HTV status and concluded that the HTV is GO for Integrated Operations,” added a second MOD update on L2.

“The demonstration objectives were met with no issues The HTV is fully operational with all systems performing nominally with two exceptions: The HTV’s gyro rate comparison FDIR may trip due to asynchronous gyro measurements between the HTV SIGIs and the RVFS (HTV Rendezvous Flight Software). This problem was not identified pre-launch, and it is only due to system timing and not to a dynamic problem.

“JAXA and NASA assessments have shown that this FDIR issue should not be a problem during Integrated Operations, except in the case of a failed capture, in which case it may lead to an unnecessary abort. To avoid this from occurring, this FDIR check will be inhibited prior to the GO for capture.

“The NASA Engineering, Ops, and Safety community do not believe inhibiting this check introduces any additional risk because there are other gyro FDIR checks that provide adequate protection.”

“The problem with the HTV SIGI and JEM GPS filters has not been resolved. However, all four systems were reset and performed well for ~24 hours, after which they are starting to show divergence issues again. JAXA has agreed to reset all four GPS filters ~14 hours before capture in order to decrease the likelihood of a reset during Integrated Operations.

“Although JAXA proposed to also inhibit the FDIR that compares the DGPS to the RGPS, NASA did not accept this proposal because JAXA would not agree to perform the equivalent FDIR comparisons on the ground. Instead, the ISSP chose to accept the risk that a filter divergence may result in a auto-abort.

“The HTV has enough consumables for 3 rendezvous attempts, so an auto-abort would not result in an end-of-mission. If this occurs, NASA and JAXA would revisit the discussion of inhibiting the FDIR in lieu of additional ground checks.”

HTV ISS Operations:

Follow arrival at the Station, the SSRMS (Space Station Remote Manipulator) played a key role, given the HTV will not dock like previous cargo ships. Instead, the SSRMS – or Canadarm2 – grappled the HTV, before robotic operations gently translated the new arrival on to the Harmony module.

Any problems – resulting in delays to the operations – during the robotics to berth the HTV on Station would have resulted in the vehicle being held in an overnight park position, while the crew prepared to attempt the docking the following day.

“If delays are expected to exceed a 10 hour work day, HTV will be taken to an Overnight Park position. Overnight park would also be used for an issue identified during PCBM inspection.

“Beta dependent park positions (4 total) designed to optimize HTV power generation. Although optimized for power generation, overnight park will result in primary battery power draw if overnight park is required.”

Further robotics will be involved when the HTV’s Exposed Pallet (EP) is handed over to the Japanese arm (JEM-RMS), which will then locate the EP on to the Exposed Facility (JEM-EF). The handoff operations between the SSRMS and JEM RMS will also be another first for the ISS.

This will be followed by Flight Day 12’s: “JEM RMS removal of EP. JEM RMS to SSRMS handoff of EP. SSRMS installation of EP into HTV.”

The 10 ton JAXA cargo vehicle is capable of supplying a total of six tons of pressurized and unpressurized cargo to the ISS at an altitude of 407 km. Pressurized cargo can be received at the rack level (an International Standard Payload Rack (ISPR)) or sub-rack level; such as Cargo Transfer Bags (CTBs).

The bulk of the HTV work on Station relates to the transfer of the vehicle’s internal cargo – involving 70 hours of soft stowage transfer and trash (from the ISS) being stowed back on the HTV – taking place between Flight Day 12 and 28, providing there are no issues with the vehicle during these procedures.

On Flight Day 29, the SSRMS will be translated back to the HTV for grappling, before the maneuver to release position.

Flight Day 30 will see the SSRMS release, ahead of the HTV’s departure burns to gain distance from the ISS.

Around two days later, the HTV will end its mission with a destructive re-entry.

L2 members: Documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.